When an announcement is made in public places such as train stations and underground shopping malls, actual voices, or recorded or synthesized voices are emitted from a speaker, for example, through a transmission channel. Such a broadcast is to transmit information to the public and, therefore, the information should desirably be correctly transmitted to the public. Sometimes information is transmitted by speeches through an outdoor loudspeaker using an emergency municipal radio communication system, or through a speaker of a municipal sound truck. At the time of a disaster, it is particularly necessary to transmit such information rightly to the public.
It is often difficult, however, to clearly hear and understand the contents of speeches in a public place such as a train station or an underground shopping mall. The reason for this difficulty is surrounding noise and acoustic transmission characteristics of the speaker. Particularly, outdoor transmission of information by speeches is adversely affected by long-path echo, wind and so on. Not only in the public places but also at home, when we listen to the radio or watch television, it is often difficult to clearly hear the speeches because of noise coming from outside and because of household noise.
The simplest solution to such a problem is to turn up (amplify) the volume. Because of the limit of output device performance, however, the volume might not be sufficiently increased, or speech signals might be distorted and become harder to hear when the volume is increased. In addition, speeches in large volume would be unnecessarily loud for neighbors and passers-by, possibly causing a problem of noise pollution.
FIG. 1 shows a typical example of prior art (Non-Patent Literature 1) for improving speech intelligibility without increasing the volume in a bad condition as described above. Referring to FIG. 1, a conventional speech intelligibility improving apparatus 30 receives input of a speech signal 32 and outputs a modified speech signal 34 with improved intelligibility. Speech intelligibility improving apparatus 30 includes: a filtering unit (HPF) 40 mainly passing high-frequency band of speech signal 32 for enhancing high frequency range of voice signal 32; and a dynamic range compression unit (DRC) 42 for compressing dynamic range of waveform amplitude of the signal output from filtering unit 40, so as to make the waveform amplitude uniform in the time direction.
Enhancement of high-frequency-range components of speech signal 32 by filtering unit 40 simulates unique utterance (Lombard speech) used by humans in a noisy environment and, hence, improvement in intelligibility is expected. The degree of enhancement of high-frequency-range components is adjusted continuously in accordance with characteristics of the input speech. On the other hand, dynamic range compressing unit 42 amplifies the waveform amplitude where the volume is locally small and attenuates the amplitude where the volume is large, so that the amplitude of speech waveform becomes uniform. In this manner, the speech becomes relatively more intelligible with indistinct sound reduced, without increasing the overall sound volume.